JPH09311130A - Method of measuring chemical oxygen demand - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stabilize an oxidizing agent for a long period by mixing cerium (III) to a cerium (IV) solution. SOLUTION: An oxidizer reagent solution is prepared by mixing cerium sulfate (III) to an aqueous solution containing 6% sulfuric acid so as to have 3.0×10<-4> M and 6.0×10<-4> M, respectively. A sample solution of 30ml or less is taken into an Erlenmeyer flask having a capacity of 100ml, 10ml of the oxidizer reagent solution is added thereto, and the mixture is heated for 30 minutes in a water bath of 100 deg.C. After the heating is completed, the content solution of the Erlenmeyer flask is transferred to a measuring cylinder having a capacity of 40ml, and regulated to 40ml with water as occasion demands. A 10-mm quartz cell is used, and the absorbance in a wavelength of 320nm is measured with reagent blank as contrast. Thus, the chemical oxygen demand(COD) can be measured with high sensitivity and high precision.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring chemical oxygen demand (hereinafter COD) in environmental water.

[0002]

2. Description of the Related Art COD is a typical index showing the degree of pollution of rivers or industrial wastewater, and is used to determine the amount of oxidizer consumed when a sample water is treated under a certain condition using a strong oxidizer. Is a value showing the amount of the oxidizable substance contained in the sample from the value converted into the amount of oxygen corresponding thereto. Regarding the COD measurement method, the amount of oxygen consumed by potassium permanganate at 100 ° C is an official method [JIS K01021
993, water supply test method (1993 edition), etc.], but unlike quantitative analysis of a specific substance, since the reaction amount is determined under specified measurement conditions,
The COD value differs depending on the type and concentration of the oxidant used, the quantification conditions such as the oxidation temperature and time, and the type of organic substance, and there are various problems such as large fluctuations in the numerical value due to cross-checking among the measurers. Especially in the low-concentration region, a slight deviation in the titration operation causes large fluctuations, making accurate measurement difficult. In order to solve these problems, the present inventor has previously attempted to quantify COD by an absorptiometric method with a smaller error and higher sensitivity, utilizing the light absorption phenomenon of an oxidant. Furthermore, the application of the flow injection analysis method that enables precise control of reaction temperature and time is also examined.
Multi-faceted experiments were conducted using three kinds of oxidants, potassium permanganate, potassium dichromate, and cerium (IV) sulfate, and some results were obtained, but the problems in COD measurement have not been fundamentally solved. [T. Kor
enaga, X. Zhou, K .; Okada, T, Mo
riwakeand S.M. Shinoda, Ana
l. Chim. Acta, 272 , 237 (199
3)].

[0003]

Therefore, as a result of intensive studies to solve the above problems, the present inventors have found and completed the present invention.

[0004]

Means for Solving the Problems The present invention is for the determination of COD by absorptiometry using cerium (IV) as an oxidant.
Is a method that enables long-term stabilization of the oxidizing agent. The oxidizing power of cerium (IV) is stronger than other oxidizing agents such as potassium permanganate and potassium dichromate, and since the reaction is a one-electron reaction, a reliable redox reaction is carried out, and permanganate and An unstable chemical species, such as dichromate, is generated in an intermediate oxidation state, so that there is no problem that an induced oxidation phenomenon occurs, and the calibration curve is linear. In addition, cerium was studied in advance, paying attention to the fact that cerium has many excellent characteristics such as not belonging to harmful metals such as manganese and chromium. Since cerium (IV) sulfate tends to cause self-decomposition in dilute sulfuric acid solution, although slightly, it causes a change in the oxidizing ability depending on the storage state of the oxidant solution, which may affect the COD measurement value. In the present invention, it was found that the oxidant solution can be stabilized by coexisting cerium (III) sulfate with cerium (IV) and constructing a redox buffer. The result of examining the stability of cerium (IV) sulfate is shown in FIG. The stability was examined from the change in the absorbance of cerium (IV) at the maximum absorption wavelength of 320 nm. (A) shows room temperature (B) stored at 4 ° C. in a refrigerator. In both cases, the absorbance decreases with time only with cerium (IV), but deterioration can be prevented by mixing cerium (III). Among them, cerium (IV): cerium (III) = 1: 2 The mixture ratio was the most stable when stored in a refrigerator at 4 ° C and could be used for 2 weeks.

[Fig. 1] In COD determination, if the type of oxidant, concentration of reagent, liquidity, heating method and heating time are different, the same values will not necessarily be obtained even with the same sample water, so various conditions are accurately determined. There is a need. 5 about reaction temperature
As a result of measuring absorbance by heating for 10 minutes in the range of 0 to 100 ° C., it was found that the reaction was best at 100 ° C. When the effect of reaction time at 100 ° C was examined, the absorbance gradually increased until 30 minutes,
No further change was seen when the reaction was continued. When the reaction temperature and the reaction time were set to 100 ° C. and 30 minutes, respectively, and the amount of cerium (IV) sulfate added was examined, a standard sample of 10 ppm had 3.0 × 10 ⁻⁴ M.
When cerium (IV) sulfate was added up to 20 ml and the reaction was carried out and the change in absorbance was observed, it increased up to 10 ml, and no further change was observed.
It was found that the addition amount of V) was 10 ml.
On the other hand, in the determination of COD with potassium permanganate, which is adopted in the official method, when chloride ions are present in the sample solution, they are disturbed because they are oxidized by potassium permanganate. Therefore, it is necessary to add silver nitrate in advance to remove chloride ions. However, according to the present invention, when an influence was examined by allowing chloride ions to coexist in a standard sample of 1 ppm, it was found that a concentration of up to 1000 ppm was allowed, and interference with potassium permanganate was found. The result is much higher than the permissible limit, and it has the characteristic that the addition of silver compounds is not necessary for ordinary river water, industrial wastewater, etc.

[0005]

EXAMPLE A quantitative operation method established by the present invention will be described. Oxidant reagent solution is used after prepared to be cerium sulfate in an aqueous solution containing 6% sulfuric acid (IV) and cerium sulfate (III), respectively 3.0x10 ^-4 M and 6.0 x ^-4 M. A sample solution of 30 ml or less is placed in an Erlenmeyer flask having a volume of 100 ml, 10 ml of the oxidant sample solution is added, and the mixture is heated in a 100 ° C. water bath for 30 minutes. After completion of heating, the content solution is transferred to an Erlenmeyer flask in a volumetric flask having a capacity of 40 ml, and if necessary, adjusted to 40 ml with water. Absorbance at a wavelength of 320 nm is measured using a 10 mm quartz cell and a reagent blank as a control. The calibration curves obtained as a result are shown in FIGS. 2 and 3. Figure 2 is COD
The calibration curve in the low concentration range from 0 to 1.6 ppm, and FIG. 3 is the calibration curve of COD from 0 to 10 ppm.

FIG. 2

FIG. 3 is 0.2 ppm. Table 1 shows the results of obtaining the respective coefficient of variation of the COD value by the official method and the COD value by the absorptiometry of the present invention at each concentration of 1.0 ppm and 10 ppm. Comparing these, it can be seen that the absorptiometric method according to the present invention has less variation in the measurement, and in particular, the absorptiometric method is more difficult than the determination by potassium permanganate titration which is difficult at a low concentration of 1.0 ppm or less. Then you can get good accuracy.

[Table 1]

[0006]

As described above, by using the present invention, highly sensitive and accurate COD measurement can be performed. The following items can be given as the peculiar effects produced by using the present invention. 1) The oxidizing power is strong, and by using cerium (IV), which is a one-electron reaction, a reliable redox reaction is performed and the accuracy is improved. 2) High sensitivity could be achieved by adopting the absorptiometry. 3) By mixing cerium (III) in the reagent solution of cerium (IV), a stable redox buffer was formed, and the reagent solution became stable and usable for a long period of time. 4) Cerium does not belong to harmful metals such as manganese and chromium, and is an environmentally friendly measurement method.
5) Allowable interference limit for chloride ion is 1000 pp
m is high, and the measurement can be performed without adding a reagent such as silver nitrate. The present invention can be said to be an excellent method as an alternative to the COD measurement by the official method, which is generally considered to have a problem in measurement accuracy, because it allows simple and reproducible quantification.

[Brief description of drawings]

FIG. 1 is a diagram showing the stability of an oxidant reagent solution according to the present invention. (A) stored at room temperature, (B) stored in a refrigerator (4 ° C).

FIG. 2 is a calibration curve according to the present invention with a COD value of 0 to 1.6 ppm.

FIG. 3 is a calibration curve for COD values of 0 to 10 ppm according to the present invention.

[Explanation of symbols]

a is a 1: 0 solution of Ce (IV) and Ce (III). b is a 1: 1 solution of Ce (IV) and Ce (III). c is Ce (IV) and Ce (III) 1: 2
Solution. d is Ce (IV) and Ce (III) 2:
Solution of 1.

Claims (1)

[Claims] 1. A method of obtaining a chemical oxygen demand by measuring an oxygen consumption amount of a reaction solution after adding an oxidant to a sample water containing an oxidizable substance, heating and reacting the same, A method for measuring a chemical oxygen demand, which comprises using a solution containing cerium (IV) and cerium (III).